In propulsion systems for self-propelled vehicles in which a mechanical transmission, which sends power to the driving wheels of the vehicle and receives at input the power supplied by an electric motor by means of a clutch coupling and a gearbox with a moderate number of gears, the continual search for construction solutions able to develop increasingly high propulsive powers, with drive units that at the same time are compact, clashes with implementation problems which are quite difficult to overcome.
In fact, it is known in general that in an electric drive unit, referable for example to the modern brushless motors type, motor performance is directly linked to the dimensions of the motor.
Consequently, to obtain an increase in the torque, the diameter of the electric motor can be increased, or the body of the motor can be lengthened; or both of those alternatives may be adopted simultaneously.
However, the practical application of such ideas is not entirely easy. In fact, considering a drive unit of a light vehicle such as a motorcycle, and, more particularly, considering a solution such as that described and illustrated in a document by the same Applicant, which integrates as one unit in a compact motor body with reduced dimensions an electric propulsion unit, a gearbox and a clutch coupling interposed between and directly connected to them, it can be seen how the freedom of construction quickly reaches limits which in practice cannot be exceeded, unless using special materials which are not commonly used for the construction of such drive units.
In fact, in the above-mentioned solution the driving shaft of the electric motor is directly connected to the clutch by means of a pair of gears. A primary gear is supported by the driving shaft. A secondary gear is in the form of a ring gear supported by a clutch housing and coupled with the primary gear. If in that configuration an attempt is made to increase the power that can be supplied by the motor in accordance with the construction approach of increasing the motor diameter, then it is automatically necessary to modify the centre-to-centre distance between the primary and secondary gears. However, an increase in the centre-to-centre distance involves increasing the primary gear tip speed. But since said speed cannot be increased indefinitely, it quickly becomes apparent that the maximum power that the electric motor can supply is subject to mechanical limits which cannot be surpassed.
Alternatively, adopting the construction approach which in contrast involves increasing the size of the motor in the axial direction of the driving shaft, the motor obtained is more powerful, but with greater axial extension, which—if used for example to propel a motorcycle—is not without limits since the increased axial extension means that there has to be a reduction in the maximum lateral angle of inclination that the motorcycle can adopt when negotiating a bend without the motor interfering with the ground.